feat(inventory): record model-zoo and start-training as on-train (#1420)

[LukasWodka]

Paired with tracebloc/release-train#25, which enrols model-zoo and start-training. repos.yml and this inventory are two files in two repos that must agree; the caller-drift audit compares them, so neither change is complete on its own.

This PR is red until its pair merges — by design

Verified locally with this edit against unchanged release-train/main:

2 drift finding(s):
- model-zoo: repos.yml says on-train=False, the inventory says True
- start-training: repos.yml says on-train=False, the inventory says True

That is the guard working. Merge release-train#25 first, then re-run audit here and merge this. The reverse order is equally red, just from the other side.

Both repos already have an fr-gate-caller.yml on developand on their prod branch, so the finding's stock warning about "joining the train without an fr-gate caller" does not apply here — the same distinction as .github#132 this morning, where the message read alarmingly and the hop was in fact gated.

A limitation worth naming rather than working around

The guard has no way to express "coordinated two-file change in flight". It compares live state against the checkout, so any cross-repo invariant is red for the window between the two merges. This is the second time today (.github#132 was the first).

A follow-up could let an inventory entry carry a pending-PR reference and treat that as a known-transient state. I have not built it, because silence during that window would be strictly worse than noise — a guard that can be told "ignore this for now" is a guard with an off switch.

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Note

Low Risk
Two boolean flags in inventory metadata; coordinated merge order with release-train#25 is the main operational note, not runtime risk.

Overview
Sets release_train: true for model-zoo and start-training in repo-inventory.yml, matching their enrollment in release-train/repos.yml (paired change in tracebloc/release-train#25).

This is inventory-only: no caller or copy requirements change in this diff. Caller-drift audit will report mismatch until both PRs land; merge the release-train side first, then re-run audit here.

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